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FreeBSD-SA-19:15.mqueuefs                                   Security Advisory
                                                          The FreeBSD Project

Topic:          Reference count overflow in mqueue filesystem

Category:       core
Module:         kernel
Announced:      2019-07-24
Credits:        Mateusz Guzik
Affects:        All supported versions of FreeBSD.
Corrected:      2019-07-23 21:12:32 UTC (stable/12, 12.0-STABLE)
                2019-07-24 12:55:16 UTC (releng/12.0, 12.0-RELEASE-p8)
                2019-07-23 21:15:28 UTC (stable/11, 11.2-STABLE)
                2019-07-24 12:55:16 UTC (releng/11.2, 11.2-RELEASE-p12)
                2019-07-24 12:55:16 UTC (releng/11.3, 11.3-RELEASE-p1)
CVE Name:       CVE-2019-5603

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I.   Background

mqueuefs(5) implements POSIX message queue file system which can be used
by processes as a communication mechanism.

'struct file' represents open files, directories, sockets and other
entities.

II.  Problem Description

System calls operating on file descriptors obtain a reference to
relevant struct file which due to a programming error was not always put
back, which in turn could be used to overflow the counter of affected
struct file.

III. Impact

A local user can use this flaw to obtain access to files, directories,
sockets etc. opened by processes owned by other users.  If obtained
struct file represents a directory from outside of user's jail, it can
be used to access files outside of the jail.  If the user in question is
a jailed root they can obtain root privileges on the host system.

IV.  Workaround

No workaround is available.  Note that the mqueuefs file system is not
enabled by default.

V.   Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Security update"

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-19:15/mqueuefs.patch
# fetch https://security.FreeBSD.org/patches/SA-19:15/mqueuefs.patch.asc
# gpg --verify mqueuefs.patch.asc

b) Apply the patch.  Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/12/                                                        r350261
releng/12.0/                                                      r350284
stable/11/                                                        r350263
releng/11.2/                                                      r350284
releng/11.3/                                                      r350284
- -------------------------------------------------------------------------

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN --summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-5603>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:15.mqueuefs.asc>
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